(1) Field of the Invention
The present invention relates to a process for purifying a nitrogen trifluoride gas. More specifically, it relates to a process for particularly removing dinitrogen difluoride from a nitrogen trifluoride gas.
(2) Description of the Prior Art
In recent years, a nitrogen trifluoride (NF.sub.3) gas is noticed as a dry etching agent for semiconductors and as a cleaning gas for CVD reactors, but the nitrogen trifluoride gas for use in these use purposes is required to have the highest possible purity.
The nitrogen trifluoride (NF.sub.3) gas can be prepared by a variety of methods, but the gas obtained through any method contains relatively large amounts of impurities such as nitrous oxide (N.sub.2 O), carbon dioxide (CO.sub.2) and dinitrogen difluoride (N.sub.2 F.sub.2). Therefore, in order to obtain the high-purity NF.sub.3 gas for the above-mentioned uses, purification is necessary.
As the purification process for removing these impurities from the NF.sub.3 gas, a process for adsorbing and removing the impurities by the use of an adsorbent such as zeolite is well known, and this process is considered to be most effective and simple (Chem. Eng. 84, p. 116, 1977). However, with regard to the purification process utilizing the adsorption, the following inconveniences appear in the case that N.sub.2 F.sub.2 is present in the NF.sub.3 gas. That is,
(1) When N.sub.2 F.sub.2 is present, the ability to adsorb the other impurities such as CO.sub.2 and N.sub.2 O deteriorates extremely.
(2) When N.sub.2 F.sub.2 is present, NF.sub.3 is also liable to be adsorbed by the adsorbent, which leads to the loss of the NF.sub.3 gas.
(3) N.sub.2 F.sub.2 which is adsorbed by the adsorbent and concentrated thereon is easily decomposed to generate heat, and in a noticeable case, explosion takes place.
In consequence, in the case that there is employed the process for adsorbing the impurities in the NF.sub.3 gas by using an adsorbent such as zeolite to remove them therefrom, it is necessary to previously eliminate N.sub.2 F.sub.2 therefrom.
As the process for removing N.sub.2 F.sub.2 from the NF.sub.3 gas, there has been heretofore known the process in which N.sub.2 F.sub.2 is reacted with an aqueous solution of KI, HI, Na.sub.2 S, Na.sub.2 S.sub.2 O.sub.3, Na.sub.2 SO.sub.3 or the like in a reaction vessel (J. Massonne, "Chem. Ing. Techn.", 41, (12), p. 695, 1969). However, this process takes a relatively long period of time to perfectly remove N.sub.2 F.sub.2, and thus not only the rather large reaction vessel but also a great deal of the agent is required.
Furthermore, another process for the removal of N.sub.2 F.sub.2 is known in which the NF.sub.3 gas containing N.sub.2 F.sub.2 is passed through a catalystic packing layer with which a reaction vessel is packed and which comprises heated metallic pieces or nets of stainless steel, carbon steel, copper, aluminum, zinc, lead, nickel, iron or the like, so that the reaction and decomposition of the gas take place on the surfaces of the metallic pieces or nets under the catalytic function of these metallic pieces or nets (Japanese Patent Publication No. 15081/1984). However, according to investigations by the present inventors, N.sub.2 F.sub.2 is reacted with the metallic pieces or nets in order to easily produce metallic fluorides on the surfaces thereof. In many cases, the thus produced metallic fluorides separate in the form of a powder from the surfaces of the metallic pieces or nets, and the packing layer and pipes of a purification apparatus are clogged with the powder disadvantageously.
In addition, the present inventors have conducted researches and have elucidated the following fact. When nickel is used as the metallic pieces, a film of fluorides is formed on the surfaces of the nickel pieces, but this film is relatively difficult to separate from the nickel surfaces. Therefore, the pipes can be prevented from clogging. However, the nickel prices covered with the fluoride film cannot react with N.sub.2 F.sub.2 any more, so that needless to say, the catalytic activity of the nickel pieces is lost. For this reason, it is necessary that operation is periodically stopped to change the inactive pieces for the new nickel pieces, which increases cost noticeably in cooperation with the use of expensive nickel.
Moreover, when the temperature of the packing layer comprising the metallic pieces is elevated up to, e.g., a temperature of 200.degree. C. or more for the purpose of heightening the removal efficiency of N.sub.2 F.sub.2, a perceptible amount of NF.sub.3 which is the main component of the gas also reacts with the metallic pieces and decomposes, so that the yield of NF.sub.3 lowers correspondingly.